In an indexing device, such as disclosed in Japan A-2 041 850, the fourth annular body exclusively along its exterior periphery forms a guide for the longitudinally slidable third annular body. The known lifting and lowering device used at this point has two hollow chambers separated from one another by the fourth annular body and filled alternately with liquid. In the case of this known arrangement, hydraulically controllable separate lifting pistons are used exclusively. Because of such pistons, the indexing device is of large structural dimensions. Also, this arrangement provides non-optimum flux of force. With the lifting of the third annular body for the purposes of engagement and mating of the crown gears with one another, a favorable moment of force arises as a result of the offset of the third annular body projecting out further than the crown gears.
An indexing device is disclosed in German 26 28 677 C2 wherein the radial bearing of the indexing table is arranged between the two coaxial first and second annular bodies. Each annular body is provided with respectively first and second crown gears. Compared with the devices known until this time, the rotation factor error of this indexing table is decreased. Thus, the annular bodies supporting the crown gears in turn can be configured as supporting bodies and are manufactured with great precision on their bearing surfaces. Precise manufacturing of these rings is also required with regard to the crown gears provided on these annular bodies. The parts required for precision of rotation are to be accommodated in a narrow construction space. Since the annular bodies with their crown gears are manufactured precisely, the precision of the relevant parts and the rotation precision of the indexing table are, for the most part, guaranteed.
With the known arrangement, in order to slide the axially slidable third annular body axially upwardly and consequently to disengage its third crown gear from the two other crown gears of the other annular body, lifting pistons of a hydraulically operable lifting device are arranged in the housing of the indexing device around the periphery at predeterminable defined distances from one another. The lifting pistons undertake the decoupling or disconnecting process from the center, controlled by an auxiliary attachment. For the lowering process and consequently for bringing the crown gears, which in this case are called "Hirth gearings", into engagement with one another, the lifting pistons of the lifting device are made pressureless. A disk spring set engages on the relevant lifting piston and takes over the axial resetting movement. With the force of the spring set, the engagement of the third crown gear of the third annular body with the other crown gears is secured for further operation of the switching device.
This type of structural arrangement requires a plurality of lifting pistons and disk spring sets in the housing. The hydraulic control, including the necessary casing or lining, is costly. Furthermore, the lifting and lowering device in the known indexing devices is incorporated in the housing. Raising of the indexing table from the device by means of a lifting tool or the like for maintenance and assembly procedures is achievable only with great difficulty, which also increases the concomitant costs.
An alternative type of indexing table device, disclosed in German 21 37 698 C3, has a lowering device which is hydraulically operable, and, for this purpose, has hollow chambers which can be filled with liquid. The hollow chambers are arranged at the end of the third annular body incorporating the third crown gear, and are controlled and filled with liquid alternately to bring the third crown gear in or out of engagement with the two other crown gears for fastening and release of the indexing table. Consequently, with this known indexing device, a hydraulically controllable separate lifting piston is used exclusively. For the lifting of the third annular body with the third crown gear, the liquid pressure on the third annular body works on this body eccentrically and in addition to the crown gears. Also, with lowering of the third annular body, a nonuniform liquid pressure arises and works on it. This leads to lower the maximum power flux. The "backfilling" arising as a result of the nonuniform introduction of force between the annular bodies with their crown gears and tolerance compensation thus becoming required is attained by dividing the third annular body into two partial rings. The partial rings in turn are provided with a third crown gear and are connected with one another through an elastically flexible intermediate element. Because this intermediate element is subjected to corrosion or attrition, the rigidity required for reliable operation of the lifting and lowering device is lost. Also, the required mating precision of the crown gears fitting into one another for securing the indexing device is not guaranteed. Therefore, the lifting and lowering device of this known indexing device is of complicated construction and is accessible only with difficulty for maintenance and assembly.